The great apes – which include chimps, orangutans, gorillas and bonobos – seem to have rich emotional lives similar to our own. But just how smart are these animals? A new generation of investigators is revealing the secret mental lives of great apes; our evolutionary next-of-kin are turning out to be far smarter than most experts ever imagined. But just as clever experiments are exposing the extraordinary abilities of great apes, new research is redefining the mental talents of our own species. Scientists are at last zeroing in on what separates us from our closest living relatives.